Process for the preparation of pilocarpine derivatives

Abstract: The invention relates to a process for the preparation of pilocarpine derivatives from a 5-formylimidazole derivative of the formula II in which R¹ is hydrogen, alkyl having 1 - 8 C atoms, or a carbocyclic radical.

Description: The invention relates to a process for the preparation of pilocarpine derivatives from a 5-formylimidazole derivative of the formula II. in which R1 is hydrogen, alkyl having 1 - 8 C atoms, or a carbocyclic radical. Of particular interest is the derivative where R 1 = methyl, which is a precursor of pilocarpine and can be converted into this in a known manner. Pilocarpine, an imidazole alkaloid, is the subject of numerous investigations on account of its varied pharmacological properties. Its outstanding pharmacological actions are diaphoretic effects, stimulation of the parasympathetic system, miotic actions and in particular applications in ophthalmology. Processes for the preparation of racemic and optically active pilocarpine are known. All these processes are characterized, however, by a comparably large number of synthesis steps, only low overall yields being achieved. In the synthesis of H. Link and K. Bernauer (Helv. Chem. Acta 55 , 1053 (1972)), racemic pilosinine is used as a precursor of pilocarpine. The starting material employed was 5-formyl-1-methylimidazole, which is easily accessible from sarcosine in a few steps. The process of Link and Bernauer comprises the Stobbe condensation of an imidazole derivative with succinic acid diester to give a monomaleate whose regioselective reduction leads to butenolide formation, from which pilosinine is produced by catalytic reduction in the last step. The disadvantage of this process is the poor course of the Stobbe condensation, where yields of at most 20% are achieved. In U.S. 5,180,837 a process for the preparation of racemic pilosinine derivatives from 5-formyl-1-methylimidazole is described. By conversion of the imidazole derivative to a thioacetal, deprotonation thereof and Michael addition to γ-crotonolactone and final desulfurization, the pilosinine derivative is accessible as a pilocarpine precursor. The disadvantage of the process described there is the low overall vield. JP-A-49116296 discloses a method for producing imidazole acetol by fermentation. The object of the present invention is to find a process for the preparation of pilocarpine derivatives from easily accessible starting materials, which proceeds in high yields and largely without by-products. This object was surprisingly achieved by the process according to the invention by means of a double olefination reaction and subsequent hydrogenation. The invention therefore relates to a process for the preparation of pilocarpine derivatives of the formula I, which is characterized in that a) the 5-formylimidazole derivative of the formula II in which R 1 is hydrogen, alkyl having 1-8 C atoms, or a carbocyclic radical, is condensed with a suitable 2-alkoxyacetic acid ester of the formula III where R 2 and R 3 in each case independently of one another are alkyl having 1-6 C atoms or Si(R 6 ) 3 and X 1 is a radical of the formula P(O)(OR 4 ) 2 , P(R 5 ) 3 or Si(R 6 ) 3 , in which R 4 , R 5 and R 6 in each case independently of one another are C 1-6 -alkyl or phenyl, to give an unsaturated carboxylic acid ester of the formula IV where R 1 , R 2 and R 3 have the meaning indicated above, b) the carboxylic acid ester is reduced to the alcohol of the formula V c) this is converted using an acid to the hydroxyketone of the formula VI d) the hydroxyketone thus obtained is converted using a suitable carboxylic acid, activated in the 2-position, of the formula VII in which X 2 has one of the meanings of X 1 , and R 7 is alkyl having 1-8 C atoms, or a carbocyclic radical, or its reactive derivatives, to an activated ester of the formula VIII where R 1 , R 7 and X 2 have the meaning indicated, e) the cyclization of the ester to the lactone ring is carried out under the action of a strong base with the formation of the dehydropilocarpine derivative of the formula IX which is converted by known processes to the pilocarpine derivative of the formula I The compounds of the formulae IV, V, and VIII are novel and thus likewise a subject of the invention. The compounds of the formula VI wherein R 1 is alkyl having 1-8 C atoms, or a carbocyclic radical, are novel and also subject of the invention. If one of the radicals R 1 to R 7 is an alkyl radical, this can be straight-chain or branched. This is thus methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, i-propyl, 1-(or 2-)methylpropyl, tert-butyl, 1-(2- or 3-)methylbutyl, neopentyl, 1-(2- ,3-or 4-)methylpentyl, 1-(2- ,3- ,4- or 5-)methylhexyl or 2-ethylhexyl (i-octyl). It is preferably methyl, ethyl or i-propyl, in particular methyl or ethyl. If R 1 and/or R 7 is a carbocyclic radical, this can be aromatic, cycloaliphatic or araliphatic. This is thus preferably phenyl, benzyl, cyclohexyl, 1-indanyl, tetrahydronaphthyl (e.g. 1,2,3,4-tetrahydro-1-naphthyl), benzocycloheptyl (e.g. 5-benzocycloheptyl), 9,10-dihydro-9-anthracenyl, 9H-fluoren-9-yl, 5-dibenzo[a,d]cycloheptyl or dihydronaphthyl (e.g. 1,2-dihydro-1-naphthyl). The abovementioned carbocyclic radicals can in each case be unsubstituted or substituted by 1 to 6 substituents selected from the group alkyl, alkoxy having 1-5 C atoms and halogen. Compounds in which R 1 is methyl, ethyl or propyl, preferably methyl, are particularly preferred. Compounds of the formula VIII, where R 7 is a straight-chain alkyl radical having 1-5 C atoms, preferably ethyl, are furthermore preferred. From the 5-formylimidazole derivative of the formula II, which is easily accessible from sarcosine methyl ester hydrochloride and dimethylamino-2-azaprop-2-en-1-ylidenedimethylammonium, as described in EP 0 366 250, using suitable 2-alkoxyacetic acid esters of the formula III, in particular phosphorus organyls, of the formula where P* is P(R 5 ) 3 or P(O)(OR 4 ) 2 , in which R 4 and R 5 in each case independently of one another are alkyl or aryl, the unsaturated esters of the formula IV can be prepared in the presence of a base. Thus, for example, phosphoranes [P*=P(R 5 ) 3 ] can be used for the olefination of aldehydes of the formula II after conversion to the corresponding ylides under the conditions of a Wittig reaction according to the processes described in Houben-Weyl, Methoden der Organischen Chemie [Methods of organic chemistry], Vol. 5/1b, p. 383, phosphonates [P*=P(O)(OR 4 ) 2 ] after formation of the anion under the conditions of a Horner Emmons reaction in analogy to the processes described e.g. in Organic Reactions, Vol. 25, John Wiley, New York, 1978, Chapter 2 or phosphine oxides in, for example, the manner described in Houben-Weyl, Methoden der Organischen Chemie, Vol. XII/1, p. 167. Preferably, phosphonates are employed, since the phosphate esters formed during the reaction are water-soluble. Suitable bases for the preparation of a reactive phosphorus ylide, phosphonate or phosphine oxide anion are, depending on the deprotonation ability of the phosphorus organyl employed, e.g. alkali metal hydroxides such as sodium hydroxide or potassium hydroxide, alkali metal carbonates such as sodium carbonate or potassium carbonate, alkoxides such as sodium methoxide, sodium ethoxide, lithium ethoxide or potassium tert-butoxide, alkali metal amides such as potassium amide or sodium amide, alkali metal organyls such as methyllithium, n-butyllithium, tert-butyllithium or phenyllithium, organic bases such as lithium diisopropylamide or sodium methylsulfinylmethylide or hydrides such as sodium hydride or potassium hydride. The reaction is expediently carried out in an inert solvent. Suitable inert solvents are preferably ethers such as diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran or dioxane, and amides such as dimethylformamide, hexamethylphosphoramide, dimethylacetamide or N-methylpyrrolidone, and also sulfoxides such as dimethyl sulfoxide or sulfolane as well as hydrocarbons such as pentane, hexane, cyclohexane, benzene or toluene. According to the reactivity of the phosphorus organyl employed, the reaction temperatures are expediently between about -10°C and about 150°C, preferably between 20 and 100°C, the reaction times between 1 and 48 hours. In the reduction of the ester of the formula IV to the alcohol of the formula V, metal hydrides are preferably employed, such as e.g. lithium aluminium hydride, sodium aluminium hydride, sodium borohydride, borohydride, sodium bis(2-methoxyethoxy)aluminium hydride, stannyl hydrides such as e.g. tributyltin hydride and in particular diisobutylaluminium hydride. Depending on the nature of the reducing agent, suitable solvents are water, alcohols such as methanol, ethanol, isopropanol, t-butanol or n-butanol, ethers such as tetrahydrofuran, dioxane or diethyl ether, and hydrocarbons such as pentane, cyclohexane, hexane, benzene, toluene or xylene or mixtures of the solvents mentioned. The reaction temperature is dependent both on the metal hydride employed and on the solvent. The reactions are preferably carried out between 0 and 100°C and the reaction times are between 15 min and 6 h. The hydrolysis of the enol ether of the formula V is preferably carried out using a mineral acid such as e.g. phosphoric acid, hydrochloric acid or sulfuric acid or N-chlorosuccinimide or ammonium chloride. The hydrolysis takes place at temperatures in the range from 0 to 100°C. The salt initially produced in the hydrolysis, of the formula VIa is converted using an alcoholic metal alkoxide solution at temperatures of 0-100°C, preferably at room temperature, to the hydroxyketone of the formula VI. Suitable metal alkoxides are potassium or sodium methoxide or ethoxide. The esterification of the alcohol of the formula VI with an acid of the formula VII, where R 7 and X 2 have the meaning indicated, R 7 is preferably methyl, ethyl, propyl, butyl or pentyl, in particular ethyl, or its reactive carboxylic acid derivatives is carried out by methods as are described, for example, in Houben-Weyl, Methoden der Organischen Chemie, Vol. VIII, pp. 359-680. In a preferred process, the preparation of the ester of the formula VIII can also be carried out by the dicyclohexylcarbodiimide (DCC) method. In the reaction the addition of an acylating catalyst has proven advantageous, since this largely suppresses the tendency to form undesired N-acylureas and thereby increases the yield. Acylation catalysts used are pyridines and pyridine derivatives, preferably aminopyridines, in particular 4-dimethylaminopyridine (DMAP) or 4-(1-pyrrolidinyl)pyridine in amounts of 3-10 mol%. The reaction takes place at temperatures between 0 and 100°C, preferably in the boiling range of the solvent used. Suitable solvents are particularly chlorinated hydrocarbons such as chloroform, dichloromethane and tetrachloromethane, and all aprotic solvents, preferably acetone, acetonitrile, dimethylformamide, dimethyl sulfoxide and ethers such as dioxane, tetrahydrofuran or diethyl ether. The cyclization of the ester to the lactone ring is carried out by conversion of the activated ester of the formula VIII to the corresponding anion. The anion is prepared analogously as described in the preparation of the unsaturated carboxylic acid ester of the formula IV. The addition of crown ethers, preferably 0.8 to 2.5 mol relative to the base employed, has proven advantageous in the cyclization reaction. Particularly when using sodium or potassium hydride or sodium or potassium carbonate as a base, the cyclization proceeds in high yields under mild conditions in the presence of 18-crown-6 or 15-crown-5. The cyclization reaction is preferably carried out at room temperature or in the boiling range of the solvent used. The pilocarpine derivative of the formula I is obtained by catalytic hydrogenation of the dehydropilocarpine of the formula IX. The catalytic hydrogenation is carried out e.g. at temperatures between about 0°C and 200°C, and at pressures between about 1 and 200 bar in an inert solvent such as e.g. an alcohol such as methanol, ethanol, isopropanol, an ether, such as e.g. diethyl ether, tetrahydrofuran (THF) or dioxane, an ester such as e.g. ethyl acetate or methyl acetate, a carboxylic acid such as acetic acid or a hydrocarbon such as cyclohexane, cyclopentane, benzene or toluene. Suitable catalysts are particularly those which allow stereoselective hydrogenation. Suitable catalysts are expediently Raney nickel or noble metals such as platinum or palladium, which can be employed in the form of their oxides e.g. PtO 2 , PdO on a support, e.g. palladium on active carbon, calcium carbonate or strontium carbonate, or in finely divided form. When using suitable, chiral reducing agents, e.g. 1,1'-binaphthyl-2,2'-dioxyaluminium hydride or chiral hydrogenation catalysts, preferably rhodium complexes of chiral diphosphines, e.g. (6,6-dimethylbiphenyl-2,2-diyl)bis(dicyclohexylphosphine), it is possible to prepare optically active pilocarpine. The process according to the invention thus allows the preparation of pilocarpine and its derivatives from imidazole derivatives in a simple manner and in high yields in a few synthesis steps from easily accessible starting substances and is thus a significant advance in the field of pilocarpine synthesis. The following example is intended to illustrate the process according to the invention without restricting it. The following abbreviations are used: THF Tetrahydrofuran DIBAH Diisobutylaluminium hydride DMAP 4-Dimethylaminopyridine DCC Dicyclohexylcarbodiimide Examples Example 1 Step I: 0.14 mol of ethyl diethylphosphonoethoxyacetate is slowly added dropwise with stirring and under inert gas to a suspension of 0.14 mol of NaH (paraffin-free) in 250 ml of abs. THF, the mixture is stirred for 1 h at 20°C and a solution of 0.093 mol of 1-methylimidazole-5-aldehyde in 100 ml of abs. THF is added dropwise. After stirring at 20°C for 10 min, the solvent is distilled off in vacuo, the residue is taken up in a little H 2 O, and the solution is acidified with 1 N HCl and washed several times with ether. The aqueous phase is rendered alkaline using 2 N NaOH with cooling (0-5°C) and extracted several times with CH 2 Cl 2 . After drying of the organic extracts with Na 2 SO 4 , the solvent is removed in vacuo. Yield: 99% of theory - the crude product is pure enough for the subsequent reaction. Step II: 122 ml of 45% DIBAH solution (= 328 mmol) are slowly added dropwise under inert gas, with stirring and ice cooling, to a solution of 137 mmol of A in 600 ml of abs. C 6 H 6 , stirring of the mixture is continued for a further 30 min at 0-5°C and 600 ml of CH 3 OH, then 100 ml of H 2 O, are slowly added. The hydroxide precipitate is filtered off with suction and washed several times with hot CH 3 OH. After drying of the combined filtrates with Na 2 SO 4 , the solvents are distilled off in vacuo and the residue is crystallized using C 2 H 5 OH. Yield: 100% of theory. The crude product is pure enough for the subsequent reaction. Recrystallization of an analytical sample from CH 3 OH/acetone: m.p. 129°C Step III: A solution of 58 mmol of B in 116.6 ml of N HCl (= 116.6 mmol) is stirred at 30-35°C for 1.5 h and concentrated in vacuo at the same temperature, and the residual HCl is removed by distillation with CHCl 3 in vacuo. After seeding, the residue crystallizes at 20°C (15 h). The crystallizate is filtered off with suction, washed with a little CH 3 OH and dried in vacuo. Yield: 86% of theory; m.p. 190°C Step IIIa: About 80-90% of the equivalent amount of NaOCH 3 solution in CH 3 OH is slowly added dropwise at 20°C with stirring and exclusion of moisture to a suspension of 21.24 mmol of C in 80 ml of CH 3 OH, in the course of which the pH of 6.5 is not to be exceeded. The solvent is distilled off in vacuo at a maximum of 30°C and the residue is purified by flash chromatography (silica gel; CHCl 3 /CH 3 OH = 17:3). Yield: 100% of theory; viscous, orange-coloured oil. Step IV: Catalytic amounts of DMAP and a solution of 21.3 mmol of D in 80 ml of CH 2 Cl 2 are added to a solution of 26.44 mmol of 2-diethylphosphonobutyric acid in 40 ml of purified CH 2 Cl 2 . After cooling to 0-5°C, a solution of 23.5 mmol of DCC in 60 ml of CH 2 Cl 2 is added dropwise and the mixture is stirred for 1 h at 0-5°C and for 2 h at 20°C. The crystallized dicyclohexylurea is filtered off with suction and the filtrate is washed with H 2 O and saturated NaHCO 3 solution. After drying of the organic phase with Na 2 SO 4 , the solvent is distilled off at 30°C in vacuo and the residue is purified by flash chromatography (silica gel; ethyl acetate/CH 3 OH = 20:3 or CHCl 3 /CH 3 OH = 30:3). Yield: 95% of theory of a viscous, orange-coloured oil. Step V (Variant a): A mixture of 5 mmol each of 80% NaH and 15-crown-5 in 50 ml of absol. toluene is stirred at 20°C under inert gas for 10 min and a solution of 5 mmol of E in 50 ml of absol. toluene is then added dropwise. Stirring is continued for a further 15 min under inert gas and the mixture is hydrolysed with a little water until phase separation is detectable. After separating off the organic phase, the aqueous layer is saturated with NaCl and extracted several times with CHCl 3 . The combined organic phases are dried with Na 2 SO 4 , the solvent is distilled off at 40°C in vacuo and the residue is purified twice by flash chromatography (silica gel; ethyl acetate/CH 3 OH = 20:3). Yield: 52% of theory; virtually colourless oil. Step V (variant b): A solution of 0.54 mmol of E in 25 ml of absol. toluene is added rapidly at 20°C with stirring and under inert gas to a mixture of 3.21 mmol of K 2 CO 3 and 6.42 mmol of 18-crown-6 in 20 ml of toluene. Stirring of the mixture is continued for a further 15 min, a little water is added until phase separation is detectable and the organic phase is separated off. The aqueous phase is saturated with NaCl and extracted several times with CHCl 3 . After drying of the combined organic phases with Na 2 SO 4 , the solvents are distilled off in vacuo at 40°C and the residue is purified by flash chromatography (silica gel; ethyl acetate/CH 3 OH = 20:3). Yield: 52% of theory Step VI: 1.36 mmol of F in 15.5 ml of CH 3 OH are hydrogenated for 5 h at 50 bar and 60°C using 210 mg of Pd/carbon (10%). After filtering off the catalyst and distilling off the solvent at 30°C in vacuo, the oily residue (about 250 mg) is treated with 10 ml of 1 N HCl and the mixture is stirred for 3 h at 20°C. The hydrochloric acid is distilled off in vacuo at 35-40°C, the oily residue is taken up in a little CH 3 OH and ether is added. The precipitate is recrystallized from CH 3 OH/ether. Yield: 73% of theory; m.p. 210°C. NMR spectrum identical with that of the (+)-enantiomer HCl.

Claims: Process for the preparation of pilocarpine derivatives of the formula I, characterized in that a) the 5-formylimidazole derivative of the formula II in which R 1 is hydrogen, alkyl having 1-8 C atoms, or a carbocyclic radical, is condensed with a suitable 2-alkoxyacetic acid ester of the formula III where R 2 and R 3 in each case independently of one another are alkyl having 1-5 C atoms or Si(R 6 ) 3 and X 1 is a radical selected from the formulae PO(OR 4 ) 2 , P(R 5 ) 3 and Si(R 6 ) 3 , in which R 4 , R 5 and R 6 in each case independently of one another are C 1-6 -alkyl or phenyl, to give the unsaturated carboxylic acid ester of the formula IV where R 1 , R 2 and R 3 have the meaning indicated above, and b) the carboxylic acid ester is reduced to the alcohol of the formula V c) this is converted using an acid to the hydroxyketone of the formula VI d) the hydroxyketone thus obtained is converted using a suitable carboxylic acid, activated in the 2-position, of the formula VII in which X 2 has one of the meanings of X 1 , and R 7 is alkyl having 1-8 C atoms or a carbocyclic radical, or its reactive derivatives, to an activated ester of the formula VIII where R 1 , R 7 and X 2 having the meaning indicated, e) the cyclization of the ester to the lactone ring is carried out under the action of a strong base with the formation of the dehydropilocarpine derivative of the formula IX which is converted by known processes to the pilocarpine derivative of the Compounds of the formula IV in which R 1 , R 2 and R 3 have the meaning indicated in Claim 1. Compounds of the formula V in which R 1 and R 3 have the meaning indicated in Claim 1. Compounds of the formula VI in which R 1 is alkyl having 1-8 C atoms, or a carbocyclic radical. Salts of the formula VIa in which R 1 has the meaning indicated in Claim 1, and Y - is a halide, sulfate or phosphate ion. Compounds of the formula VIII in which R 1 , R 7 and X 2 have the meaning indicated in Claim 1. Process according to Claim 1, characterized in that the hydroxyketone of the formula VI is reacted with an acid of the formula VII or its reactive derivatives and dicyclohexylcarbodiimide in the presence of an acylating catalyst. Process according to Claim 7, characterized in that the acylating catalyst is 4-dimethylaminopyridine or 4-(1-pyrrolidinyl)pyridine. Process according to Claim 1, characterized in that the cyclization of the activated ester of the formula VIII takes place in the presence of a crown ether. Process according to Claim 7, characterized in that 0.8-2.5 mol of the crown ether are employed, relative to the base used. Compounds according to Claims 2-6, characterized in that R 1 is a methyl radical. Compounds according to Claim 6 or 11, characterized in that R 7 is an ethyl radical.

Inventors: REIMANN EBERHARD PROF DR, REIMANN, EBERHARD, PROF. DR.
Applicants: MERCK PATENT GMBH
Application year: 1994
Main classification: C07D405
IPC codes: C07D405, A61P27, C07D233, A61K31